1. Field
This invention relates in general to heat exchangers and to apparatus for preventing vibration within the tube bend region of a U-tube heat exchanger and more particularly to apparatus and methods for the spacing of anti-vibration bars in the tube lanes between tube columns in the U-bend region of such heat exchangers.
2. Related Art
U-shaped heat exchangers are commonly employed in pressurized water nuclear reactor steam generators. A nuclear steam generator generally comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and a hemispheric channel head to form a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside, and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the inside of the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator secondary side through a feedwater nozzle which is connected to the feedwater ring inside the steam generator. Upon entering the steam generator, the feedwater mixes with water returning from moisture separators positioned above the U-tube bundle, referred to as the recirculation stream. This mixture, called the downcomer flow, is conducted down the annular chamber adjacent to the shell between the shell and the wrapper until the tube sheet near the bottom of the annular chamber causes the water to reverse direction, passing in heat exchange relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat exchange relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the water surrounding the tubes, causing a portion of the water outside the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate any entrained water from the steam, and the steam vapor then exits the steam generator and is circulated typically through electrical generating equipment to generate electricity in a manner well known in the art.
The portion of the steam generator primarily including the bundle of U-shaped tubes and the channel head is typically referred to as the evaporator section. The portion of the steam generator above the U-shaped tubes that includes the moisture separators is typically referred to as the steam drum. Feedwater enters the steam generator through an inlet nozzle which is disposed in the upper portion of the cylindrical shell. The feedwater is distributed and mixed with water removed by the moisture separation and then flows down the annular channel surrounding the tube bundle.
The U-tubes are supported at their open ends by conventional means whereby the ends of the tubes are seal welded to the tube sheet which is disposed transverse to the longitudinal access of the steam generator. A series of tube support plates arranged in an axial spaced relationship to each other are provided along the straight portion of the tubes in order to support the straight section of the tubing. An upper tube support assembly is utilized to support the U-shaped portion of the tubes of the tube bundle. The upper assembly comprises a plurality of retaining rings arranged around the outside of the tube bundle in spaced relationship to each other.
The retaining rings, like the tube support plates, are arranged substantially transverse to the longitudinal access of the steam generator. Each retaining ring is generally an oval shape which coincides with the outer periphery of the tube bundle at the particular location of the retaining ring. Thus, the size of the oval of the retaining rings decreases with the distances toward the upper end of the tube bundle. The uppermost retaining ring, therefore, is relatively small inasmuch as it is located at the uppermost portion of the tube bundle where the shape of the tube bundle is rapidly converging.
Each of the retaining rings is connected to a plurality of anti-vibration bars which are typically disposed between each column of the U-shaped portion of the tubes. In some steam generators, the anti-vibration bars comprise a bar bent into a V-shaped configuration such that two legs are formed with an angle therebetween. The V-shaped bars are inserted between successive columns of steam generator flow tubes. The V ends of the bars are inserted between the flow tubes; the free ends of the bars are welded to opposite sides of the appropriate retaining ring. In this manner, each of the tubes of the tube bundle is supported along the length of the curved or U-shaped portion at a number of spaced locations by an anti-vibration bar. This arrangement provides line support and yet allows the feedwater to flow around and between the curved portion of the steam generator tubes. In other words, the anti-vibration bars provide support and do not substantially interfere with the flow of the recirculation water.
The anti-vibration bars are intended to prevent successive vibrations of the individual tubes of the entire tube bundle. The vibrations in question are caused by the flow of water and steam past the flow tubes. These flow-induced vibrations can potentially damage the flow tubes. It is well known that the U-shaped portion of the tube bundle is more severely affected by the vibrations, and, because of the U-bend configuration, more difficult to adequately support in order to eliminate the vibrations. While the advent of the anti-vibration bars has materially reduced the magnitude and presence of vibrations, they have not in all cases completely eliminated damage which is caused by vibrations.
The mechanical aspects of the curvature of the U-bend portion of the tubes of the tube bundle are major obstacles to finding a mechanical solution to this problem. The U-shaped tubes of the tube bundle have dimensional tolerances associated with their outer diameter. There are also variations caused by ovalization of the tubes as a result of the bending. Furthermore, the spatial relationship between adjacent tubes is a variable, albeit, within design limits. Thus, there is a dimensional tolerance associated with the nominal spacing between the steam generator tubes. There is a dimensional tolerance associated with the outer dimensions of the anti-vibration bars. The combination of these tolerance and dimensional variances prevent the elimination of undesirable gaps between the vibration bars and the tubes of the steam generator. Any large gaps are undesirable because they allow vibration of the tubes and relative motion between the tubes and the anti-vibration bars. The relative motion can cause wear and subsequent damage or failure of the tubes. Therefore, it is important to control the spacing between the tubes and the anti-vibration bars for vibration control purposes and additionally to limit the pressure that the anti-vibration bars exert upon the tubes in that too much pressure can damage the tubes. Accordingly, it is an object of this invention to provide means to control the spacing between the anti-vibration bars and the tubes before the vibration bars are welded at their ends to the retaining rings during manufacture of the steam generator.
It is a further object of this invention to automate the process of spacing the anti-vibration bars from the tubes in the tube lanes of a tube column before the anti-vibration bars are welded at their ends to a corresponding retaining ring.
It is an additional object of this invention to establish a consistent spacing between the flow tubes and the adjacent anti-vibration bars.
Another object of this invention is to provide means for creating a record of the as built spacing of the vibration bars from the flow tubes after vibration bar ends have been welded to the corresponding retaining rings.